This invention relates to stable, aqueous latexes having biocidal properties and to methods for their preparation. Specifically, the present invention provides a combination of biocides to provide antimicrobial protection that individual biocides cannot provide. The combination is especially suitable where latexes have been pH adjusted into a pH range that provides thermostability but provides a more susceptible environment for microbial attack.
Aqueous dispersions of polymers, which are referred to as “latexes” in the art, are generally known to be useful, both alone and in a variety of coatings, including films, polishes, varnishes, paints, inks, and adhesives. A wide variety of latexes of various homopolymeric and copolymeric compositions (such as styrene-butadiene copolymers, acrylic homopolymers and copolymers, vinylidene chloride homopolymers and copolymers, etc.) have been developed having specific chemical and/or mechanical properties for particular end use applications.
For example, the stabilized latex emulsion described in U.S. Pat. Nos. 5,081,166 and 4,894,397 is a stabilized latex emulsion produced by: (i) reacting latex-forming monomers under emulsion-polymerization reaction conditions to form a hydrophilic first-stage polymeric precursor; (ii) contacting the first-stage polymeric precursor with at least one hydrophobic latex-forming monomer under emulsion-polymerization reaction conditions to form a hydrophobic second-stage polymeric precursor such that a portion of the second-stage hydrophobic polymeric precursor partitions into the first-stage hydrophilic polymeric precursor thereby producing an inverted core-shell latex emulsion polymeric precursor; and (iii) adjusting the pH of the inverted core-shell latex emulsion polymeric precursor to dissolve at least a portion of the first stage hydrophilic polymeric precursor thereby producing a stabilized latex emulsion including a continuous aqueous phase containing the first-stage hydrophilic polymeric precursor and a discontinuous phase containing discrete, stabilized particles of the second-state hydrophobic polymeric precursor. The resulting stabilized emulsion can be used to produce a variety of coatings including films, polishes, varnishes, paints, inks, and adhesives.
In the process of U.S. Pat. Nos. 5,081,166 and 4,894,397, the step of adjusting the pH of the inverted core-shell latex emulsion for dissolving the hydrophilic polymer is particularly advantageous as it serves to produce a stabilized latex emulsion. If acidic functional group monomers are selected for the first-stage polymer used in producing the inverted core-shell latex, addition of a suitable base is appropriate for adjusting the pH of the inverted core-shell latex emulsion toward or to a neutral pH.
It has been discovered, however, that raising the pH has drawbacks in certain applications of latexes. Specifically, raising the pH has the effect of reducing the number of acid groups which are known to have bactericidal or bacteriostatic properties. In environments where the latex is prone to bacterial growth, the elimination of acid groups may lead to bacterial growth in the latex.
Therefore, it would be beneficial to provide a latex having the dual advantages of stabilization through pH adjustment and bactericidal or bacteriostatic properties.